That was surprising, because baby booms often parallel economic booms, and last year was a period of low unemployment and a growing economy.

But other factors are likely at play, analysts said.

One may be shifting attitudes about motherhood among millennials, who are in their prime child-bearing years right now. They may be more inclined to put off child-bearing or have fewer children, researchers said.

Another may be changes in the immigrant population, who generate nearly a quarter of the babies born in the United States each year. For example, Asians are making up a larger proportion of immigrants, and they have typically had fewer children than other immigrant groups.

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Also, use of IUDs and other long-acting forms of contraception has been increasing.

The Centers for Disease Control and Prevention report also found:

► The rate of births to women ages 15 to 44, known as the general fertility rate, sank to a record low of about 60 per 1,000.

► Women in their early 40s were the only group with higher birth rates in 2017, up 2 percent from the year before. The rate has been rising since the early 1980s.

► The caesarean section rate rose by a tiny amount after having decreased four years.

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► Rates of preterm and low birth weight babies rose for the third straight year.